Within `Behaviour::record_received` the exponentially decreasing
expiration based on the distance to the target for a record is
calculated as following:

1. Calculate the amount of nodes between us and the record key beyond
the k replication constant as `n`.

2. Shift the configured record time-to-live `n` times to the right to
calculate an exponentially decreasing expiration.

The configured record time-to-live is a u64. If `n` is larger or equal
to 64 the right shift will lead to an overflow which panics in debug
mode.

This patch uses a checked right shift instead, defaulting to 0 (`now +
0`) for the expiration on overflow.

Extract right shift into isolated function and replace complex
regression test with small isolated one.